Implantable medical devices include, among other things, cardiac rhythm management (CRM) devices such as pacers, cardioverters, defibrillators, cardiac resynchronization therapy (CRT) devices, as well as combination devices that provide more than one of these therapy modalities to a subject. For example, an implantable defibrillator/pacer is typically configured as an implantable defibrillator with backup pacing capability. Such devices are intended to serve patients having a history of previous ventricular or atrial tachyarrhythmia episodes. Ventricular arrhythmias include ventricular tachyarrhythmia (VT) and dangerous and life-threatening ventricular fibrillation (VF), referred to collectively herein as VT/VF. VT/VF is typically treated with antitachyarrhythmia pacing (ATP) therapy or a defibrillation countershock.
A cardiac rhythm management device's detection scheme for a particular cardiac arrhythmias is typically characterized by its “sensitivity” and “specificity.” Sensitivity generally refers to the ability of the detection scheme to effectively detect an abnormal heart rhythm (e.g., VT/VF) that the physician desires the cardiac rhythm management device to treat. The sensitivity can be expressed as follows:Sensitivity=True Positives/(True Positives+False Negatives)  (Eq. 1)Specificity generally refers to the ability of the detection scheme to avoid improperly treating rhythms (e.g., sinus tachycardia) that the physician determines that the device should not treat. The specificity can be expressed as follows:Specificity=True Negatives/(True Negatives+False Positives)  (Eq. 2)
For example, if the rhythm to be detected is VT/VF, then a true positive would occur when a particular rhythm is VT/VF and the detection algorithm correctly declares it as VT/VF. A false negative would occur when the rhythm is VT/VF and the detection algorithm erroneously declares it as not VT/VF. A false positive would occur when the rhythm is anything but VT/VF (e.g., normal sinus rhythm (NSR), sinus tachycardia, atrial fibrillation, atrial flutter, electrical noise, e.g., due to mypotentials, electromagnetic interference (EMI), a loose set screw for a leadwire, a broken leadwire, etc.) and the detection algorithm erroneously declares it as VT/VF. A true negative would occur when the rhythm is anything but VT/VF (e.g., normal sinus rhythm (NSR), sinus tachycardia, atrial fibrillation, atrial flutter, electrical noise, e.g., due to mypotentials, electromagnetic interference (EMI), a loose set screw for a leadwire, a broken leadwire, etc.) and the detection algorithm correctly declares it as not VT/VF.
Ideally, a cardiac rhythm management device would have both 100% sensitivity and 100% specificity. However, it is well known in the art that for practical cardiac rhythm management devices, there exists a tradeoff between sensitivity and specificity, such that no practical detection scheme can obtain the ideal. As discussed above, existing implantable defibrillator/pacers are typically targeted toward patients with a history or high risk of life-threatening VT/VF episodes. Because of the severe (indeed life-threatening) consequences of failing to treat a VF episode, for example, existing defibrillator/pacers are typically configured to maximize sensitivity to VT/VF. To accomplish this, such devices typically sacrifice specificity. That is, they will generally tolerate the delivery of inappropriate countershocks (i.e., a lower specificity) if needed to maintain the desired high sensitivity. This ensures that virtually no VF episode will go untreated. It is true that many such defibrillator/pacers go through great lengths to improve the specificity to avoid inappropriately delivering a painful countershock to the patient. Still, such specificity enhancements typically are a secondary consideration—specificity cannot be increased if doing so would cause an appreciable number of VF episodes to go untreated—the potential consequences are too severe, particularly for a tachyarrhythmia patient population.
Bradycardia patients, on the other hand, typically receive a pacer without defibrillation capability, as presently called for by standard clinical, health insurance, and government reimbursement guidelines. However, a significant number of pacemaker patients die from VF and polymorphic VT—even if no such previous episodes have been diagnosed. Such patients are ineligible for a defibrillator/pacer device, however, they could benefit from defibrillation therapy. As discussed above, however, existing defibrillator/pacer devices, however, are typically designed as defibrillators with backup pacing capability—they are not intended for bradycardia patients and, moreover, because of the needs of the tachycardia patient population for which they are designed, they are not well suited for bradycardia patients.
In sum, the present inventors have recognized a need in the art for improved cardiac rhythm management devices having both pacing and defibrillation therapies.